For the elucidation of the conformation and interactions of proteins in solution, nuclear magnetic resonance (NMR) spectroscopy in the method of choice. To study strong and selective peptide-protein binding, as a model for horone peptide-receptor interactions the ribonuclease S' (SR') system has been chosen, particularly since it provides a simple enzymatic test of biological viability. Selectively 13C-enriched peptides of the (1-15) amino-terminal portion of ribonuclease were synthesized, and resolved resonances of individual 13C atoms in the RS' complex were monitored. Both at natural abundance (1.1%) and with 13C enrichment the 13C NMR technique has been extended to the observation of Met, His, Ala, Asp and Tyr residues in several proteins.